1. Field of the Invention
The present invention relates to an optical recording and/or reproducing apparatus which focuses a beam from a light source for an optical disk or the like in the form of a fine optical spot on an optical recording medium to record and/or reproduce information. More particularly, the invention relates to the shape and optical layout of a beam splitter in a fixed optical unit in a separate optical system in which a light source portion and photosensors are stationary and only optical elements such as an objective lens and a reflection mirror are mounted in a movable unit to be moved in the direction of a disk radius.
2. Related Background Art
Recently, optical memories for recording and/or reproducing information with a semiconductor laser beam have vigorously been introduced commercially as a high density recording memory. Especially, magneto-optical recording and/or reproducing apparatus in which information is rewritable are considered to be promising. The magneto-optical recording and/or reproducing apparatus magnetically record information utilizing a local temperature rise in a magnetic thin film upon irradiation in the form of a spot with a laser beam and reproduce the information by the magneto-optical effect (Kerr effect).
Here, described is an optical system for an optical head in the magneto-optical recording and/or reproducing apparatus as having been proposed heretofore.
In FIG. 1, a beam from a semiconductor laser 1 is collimated by a collimator lens 2 and the thus 10 collimated beam enters a polarization beam splitter 3. A plane of incidence of the polarization beam splitter 3 is inclined at a predetermined angle relative to the beam from the collimator lens 2, whereby the beam from the semiconductor laser 1 having an anisotropic light intensity distribution is shaped into a beam having an isotropic light intensity distribution. The direction of polarization of the beam from the semiconductor laser 1 is selected in the direction of an arrow 4. The beam passing through the polarization beam splitter 3 is focused as a fine optical spot on a magneto-optical disk 6 by an objective lens 5. An arrow 7 represents the direction of information tracks on the magneto-optical disk 6.
To keep the quantity of light from the semiconductor laser 1 constant, the apparatus performs such a feedback control that a photosensor 8 receives the beam not going to the magneto-optical disk 6 but reflected by the polarization beam splitter 3 and that an output from the sensor 8 is fed back to the semiconductor laser 1.
When the beam is reflected by the magneto-optical disk 6, the plane of polarization of the reflected beam is slightly rotated. The reflected beam again enters the objective lens 5 and then is reflected by the polarization beam splitter 3. The thus reflected beam enters a half wave plate 9, where the plane of polarization is rotated 45.degree.. Then, a condenser lens 10 and a cylindrical lens 11 focus the beam on photosensors 13, 14.
Located before the photosensors 13, 14 is a polarization beam splitter 12, which transmits a component of p-polarized light toward the photosensor 13 and which reflects a component of s-polarized light toward the photosensor 14. Since the aforementioned rotation of the polarization plane is converted into a difference between quantities of light reaching the two photosensors 13, 14, a differential output thereof can provide a magneto-optical signal.
Also, the cylindrical lens 11 works to change the shape of the optical spots on the photosensors 13, 14 for detecting the magneto-optical signal in accordance with a focused state of the optical spot on the magneto-optical disk 6, whereby a focusing error signal can be obtained. Similarly, light intensity distributions of the optical spots on the photosensors 13, 14 change depending upon track deviation of the optical spot on the magneto-optical disk 6, whereby a tracking error signal can be obtained.
For recording information on the magneto-optical disk 6, a magnetic head 30 applies a magnetic field to a portion where the optical spot is focused by the objective lens 5, so as to record information.
Meanwhile, many of recently commercially available magneto-optical recording and/or reproducing apparatus employ an arrangement of a separate optical system in which the light source portion and photosensors are arranged to be stationary and only optical elements such as the objective lens and the reflection mirror are mounted in a movable unit to be moved in the direction of the disk radius, in order to reduce the time of access to a target information track.
An optical head employing the conventionally proposed separate optical system is next described with respect to FIG. 2. Elements having the same functions as those in FIG. 1 are denoted by the same reference numerals.
The optical head in FIG. 2 is composed of an optical system movable unit 16 and an optical system stationary unit 18. An optical system of the movable unit is light in weight and compact as being composed of a reflection mirror 15, an objective lens 5, and focusing and tracking actuators, and designed as to give access to a target information track within a short time. The movable unit is actually so arranged as to be rotated 90.degree. about the optical axis perpendicular to the arrow 7 representing the direction of information tracks on the magneto-optical disk 6.
Now, for apparatus using a magneto-optical disk with a diameter of 90 mm, the stationary unit 18 also needs to be constructed in a compact structure because of the limit of dimensions of the apparatus. Thus, the beam collimated by the collimator lens 2 is arranged to be reflected by a mirror 17 to enter the polarization beam splitter 3. By this arrangement, the length of the stationary optical system along the radial direction of the disk can be made shorter to some extent than that of the optical system shown in FIG. 1.
Nevertheless, the semiconductor laser 1 and the photosensor 8 are juxtaposed in the radial direction of the disk, which makes it difficult to further decrease the length of the stationary unit 18. It is similarly difficult in the case with such an arrangement that the mirror 17 is inverted and the semiconductor laser 1 is arranged in the same direction as the photosensors 13, 14.
Further, if the separate optical system should be so arranged that the beam emitted from the stationary unit 18 and then reaching the movable unit 16 was not in parallel with the radial direction of the disk, a tracking offset would occur with movement of the movable unit 16. Thus, the system needs a mechanism for adjusting an inclination of the mirror 17, which is against a desire to make the stationary unit more compact and also raises the production cost.